Worldwide, approximately 31% of adults and 80% of adolescents (13-15 years of age) are physically inactive.1 Across countries and even within countries, disparities exist in the prevalence of physical inactivity.1,2 In the US, rates are highest among women and racial/ethnic minorities.3 Reasons for these disparities are unknown but studies show that physical activity (PA) is influenced by factors at the individual-, social-, and environmental-levels, as posited by ecological models of health behavior4. To date, most ecological studies of PA have focused on associations of factors at a single level of influence, while few have tested interactions across levels (e.g., between factors at the social- and environmental-levels). Evidence of cross-level interactions in diverse settings and populations may help explain some of the variations observed in the associations between environmental factors and PA. Furthermore, while multilevel interventions for increasing PA are becoming more available, few studies have examined intervention-mediated effects via changes to targeted environmental constructs. Mediation analyses using longitudinal data can help advance understanding of mechanisms of PA behavior change in the context of a multilevel intervention. The proposed research can help address the aforementioned knowledge gaps in PA research. Analyses will use data collected among adults in the International Prevalence Study on Physical Activity, US adolescents in the Teen Environment and Neighborhood study, and a cohort of Latina women participating in a PA multilevel intervention - Faith in Action. The Specific Aims are to: 1) Assess interactions between socio-demographic and perceived environmental factors on reported total PA among an international sample of urban adults; 2) Test interactions between psychosocial and objectively-assessed environmental factors on reported active travel to/from school and neighborhood leisure-time PA among US adolescents; and 3) Examine the effects of a multilevel intervention on perceived environmental constructs and whether changes to these constructs predict changes in Latinas' accelerometer-assessed and reported leisure-time moderate-to-vigorous PA at 12- and 24-months post-intervention. The proposed Specific Aims are consistent with the goals of the National Cancer Institute (NCI) and relate to NCI's scope of work in cancer prevention population-based research and environmental epidemiology. To accomplish the proposed research, the applicant will execute a training plan that enhances her knowledge and skills in health behavior theories and research, physical activity epidemiology and cancer prevention research, study design and data analyses, and a wide range of built environment measures. The applicant will conduct the proposed research with support from a dedicated mentorship team in a resource-rich environment at the University of California, San Diego and San Diego State University. Upon successful completion of this fellowship, the applicant will be well prepared to meet her long- term goals of becoming an independent researcher in PA promotion and cancer prevention.